Industrial and medical particle accelerators such as electron beam accelerators enjoy an annual worldwide market of approximately many millions of dollars. They are used in applications ranging from product sterilization of e.g. medical instruments and food containers, to material modification such as tire vulcanization, printing ink curing, plastics cross-linking and paper manufacture, to electron-beam welding of thick-section plates in e.g. automobile manufacture and to medical applications including radiation therapy. Other applications include chemical-free municipal water sterilization and boiler flue gas treatment to remove sulfur and nitrogen oxides from the effluent gases and create fertilizer in the process. Linear particle accelerators in particular may also be used as an injector into a higher energy synchrotron at a dedicated experimental particle physics laboratory.
There are generally three major types of particle accelerators:                Electrostatic accelerators in which the particles are accelerated by the electric field between two different fixed potentials. Examples include the Van der Graff, Pelletron and Tandem accelerators.        Radio-frequency (RF) based accelerators in which the electric field component of radio waves accelerates particles inside a partially closed conducting cavity acting as a RF resonator.        Induction-based accelerators in which pulsed voltage is applied around magnetic cores to thereby induce an electric field for accelerating the particle beam.        
Electrostatic accelerators such as the classical Van der Graff accelerators have been used for years, and are still in use in e.g. experimental particle and/or ion beam installations.
Present RF-based accelerator technology normally uses a variety of high voltage generators which are enclosed in pressurized gas tanks. The two dominant designs are based on the Dynamitron (Radiation Dynamics Inc, RDI) and the Insulated-Core Transformer or ICT (Fujitsu of Japan). The Dynamitron is powered by ultrasonic radio frequency oscillations from a vacuum tube generator. The ICT is powered by A.C. from the conventional power line. Another high power machine, the Rhodotron, is also commercially available on the market. However, all of these machines suffer from one or more of the disadvantages of using high-voltage generators, dangerous and heavy high pressure tanks, and potentially toxic and expensive gases.
In the early 1960's a so-called Linear Magnetic Induction (LMI) Accelerator was designed by Nicholas Christofilos of the U.S. Government's Lawrence Livermore National Laboratory (LLNL). At that time, the laboratory was named “Lawrence Radiation Laboratory” or LRL. This accelerator design was based on the use of a large number of toroidal (doughnut-shaped) magnetic cores, each core being driven by a high voltage pulse generator at several tens of kilovolts (kV) (using a spark-gap switch and a pulse-forming network or PFN) to generate an accelerating potential of several hundred kV to several megavolts (MV) to accelerate a high-current beam of charged particles.
A key feature of this type of accelerator is that it, like all Linear Accelerators (LINACs), has an outer surface which is at ground potential. The voltages which drive the individual cores all appear to add “in series” down the central axis, but do not appear anywhere else. This means the accelerator does not radiate electromagnetic energy to the “outside world” and is easy to install in a laboratory as it needs no insulation from its surroundings. An 800 kV LMI accelerator, the ASTRON linear accelerator, was built at LLNL in the late 1960s [1], and was used for electron-beam acceleration in fusion experiments. A larger LMI machine (FXR, Flash X-Ray) was built in the 1970s, and used for accelerating an electron beam pulse into an x-ray conversion target. The FXR accelerator was used for freeze-frame radiography of explosions.
The basic idea of this so-called Linear Magnetic Induction (LMI) Accelerator is schematically illustrated in FIG. 1. The LMI accelerator of FIG. 1 is built around a set of toroidal magnetic cores arranged so their central holes surround a straight line, the so-called central beam axis, along which the particle beam is to be accelerated. Each magnetic core has a high-voltage drive system comprising a high-voltage pulse Forming Network (PFN) and a high voltage switch such as a spark gap switch. For simplicity, only one drive section is shown in FIG. 1.
The high-voltage switch is typically a plasma or ionized-gas switch such as a hydrogen thyratron tube that can only be turned on but not turned off. Instead, the PFN is required to create the pulse and deliver power in the form of a rectangular pulse with a relatively fast rise and fall-time as compared to the pulse width. The PFN normally discharges in a traveling-wave manner, with an electrical pulse wave traveling from the switched end to the “open circuited” end, reflecting from this open circuit and returning toward the switched end, extracting energy from the energy storage capacitors of the PFN network as it travels and “feeding” the energy into the core section. The pulse ends when the traveling wave has traversed the PFN structure in both directions and all the stored energy has been extracted from the network. The PFN voltage before switching is V, and the voltage applied to the primary side of the pulse transformer is V/2 or a bit less. If a component in the PFN fails, it is necessary to re-tune the PFN for optimal pulse shape after the component is replaced. This is laborious and dangerous work, as it must be done with high voltage applied to the PFN. Besides, if a different pulse width is needed, it is necessary to replace and/or re-tune the entire PFN structure. The high-voltage PFNs and switches also suffer from disadvantages with respect to reliability and safety.
Several companies have built accelerators based on the early ASTRON design. The designs used to drive the accelerators are based on spark gap or thyratron switches in combination with the cumbersome high-voltage PFN networks, and so are not cost-competitive with the RF-based designs such as the Dynamitron and the ICT.
There are also modern designs which are based on solid-state modulator systems that convert AC line power into DC power pulses, which in turn are transformed into radio frequency (RF) pulses that “kick” the particles up to the required energy levels [2].
Other examples of solid-state modulators that can be used for driving RF-based systems are disclosed in [3-5].
LLNL has also presented compact dielectric wall accelerators (DWA) and pulse-forming lines that operate at high gradients to feed an accelerating pulse down an insulating wall, with a charged particle generator integrated on the accelerator to enable compact unitary actuation [6]. Other examples based on DWA and/or Blumlein accelerator technology are described in [7-8].
There is a general need for improvements in particle accelerator design with respect to one or more of the issues of cost-effectiveness, reliability, on-line availability, size, energy-consumption and safety.